Polyimides are plastics with many technical uses. Various methods are known for producing polyimides, particularly where the preparation takes place in a two-stage polycondensation reaction of a tetracarboxylic acid dianhydride with a diamine in solution. In the first stage, a polyamido carboxylic acid is obtained, which in the second stage is transformed thermally--at temperatures up to 400.degree. C.--or chemically--with dehydrating agents--under dehydration into the polyimide. According to this method, both purely carbonorganic polyimides as well as siliconimides can be produced. U.S. Pat. Nos. 3,325,450, 3,847,867 and 4,558,110, for example, describe the preparation of siliconimides from organic anhydrides and amino functional silicones. The preparation of siliconimides from anhydride-functional silicones and organic dismines is known from the German Published Patent Application 38 03 088, U.S. Pat. Nos. 4,404,350 and 4,794,153.
Another way to produce polyimides is by reacting isocyanates with carboxylic acid anhydrides while separating carbon dioxide (see, for example, U.S. Pat. No. 4,021,412). Lewis acids and tertiary amines catalyze this reaction, but the admixture of small quantities of alcohol or water is also believed to have an advantageous effect (see German Published Patent Application 19 58 900 as well as Polym. Prepr., vol. 9 (1968), pp. 1581 to 1586).
German Published Patent Application 37 01 249 discloses a method for producing alkoxy- and aryloxy-substituted aromatic polyimides, in which in an initial reaction aromatic diisocyanates blocked with 1,3-imidazole are produced. They are subsequently made to react in a dipolar, aprotic solvent, suitably in the presence of a catalyst, with aromatic tetracarboxylic acid dianhydrides. The high-molecular, purely carbon-organic polyimides prepared in solution in this manner, which do not exhibit any cross-linkable functions, can be processed in the solution, or rather are plastically deformable in the melt.